A. Field of the Invention
The invention relates generally to an ac-dc converter, and in particular it relates to an aircraft ac-dc converter that utilizes a cycloconverter to provide DC power for aircraft components that require DC.
B. Description of the Related Art
Variable speed constant frequency (VSCF) power generation systems are known in the aircraft power systems art for powering aircraft components. Some VSCF systems include a cycloconverter, which is a known component in the aircraft power systems art for providing AC power for an aircraft.
VSCF systems also typically include a generator machine that provides a variable speed power that is input to a frequency conversion portion (e.g., cycloconverter) of a VSCF system. One such generator machine that can generate variable frequency power for a VSCF system corresponds to a three-synchronous machine set. The three-synchronous machine set includes a main generator (MG), an exciter (EXC), and a permanent magnet generator (PMG).
Aircraft generators are typically driven by variable-speed aircraft engines, such as, for example, turbo-jet, turbo-fan, and propeller-fan engines. In such systems, the engine speed varies over the ground/flight envelope, whereby an aircraft may be taking off at one flight time period, it may be cruising at another flight time period, and it may be landing at another flight time period. The engine rotational speed is likely to be different for each of these different flight time periods. When an aircraft engine drives the generator so as to provide torque to the generator, the generator turns at a variable speed corresponding to the current rotational speed of the engine. The generator outputs a variable frequency power based on the variable engine rotational speed.
The generator is coupled to the cycloconverter in a typical VSCF system. The cycloconverter demodulates a lower frequency waveform from a higher frequency waveform, and typically it includes a static power converter that has a plurality of silicon controlled rectifiers (SCRs). The lower synthesized AC waveform output of the SCRs is passed through a filter to yield the desired constant frequency power, which typically is a 400 Hz power signal for many conventional aircraft systems.
In order to generate a high quality 400 Hz power signal to be used to drive various aircraft components, the generator must turn at a high speed in order to produce a high enough frequency to be demodulated. Typically, the generator must turn at least three or four times higher than the 400 Hz power desired. Accordingly, the engine must turn at least 1200 to 1600 revolutions per minute.
Current aircraft designs are moving to more DC power, whereby a DC power system is needed instead of an AC power system. Several approaches have been presented in order to provide DC power from a VSCF system. As described in U.S. patent Publication 2003/0142529, while a cycloconverter system has the ability to produce both AC and DC simultaneously, it does not produce clean waveforms. For such systems, the voltage regulation is accomplished by a series of magnetic amplifiers, transformers, and bridge rectifiers. Accordingly, such systems for providing DC to aircraft components are not feasible in practice, since the unclean (i.e., noisy) waveforms result in poor aircraft component performance.
In another system for providing DC power from a VSCF system, as described in U.S. Pat. No. 4,587,436, a bridge rectifier is placed in the generator housing to rectify the three-phase variable-frequency AC power to a DC voltage, such as to 270 VDC. However, such systems require the inverting of a large percentage of the power for motor loads and for the large amount of 400 Hz equipment, thereby incurring significant weight requirements that are clearly disadvantageous for aircraft designs that operate under the maxim that “lighter is better.”
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.